That could mean when the next stitch on your needle is a knit, purl it, if it looks like a purl, knit it. That will make seed stitch, but that may not be what it means - it may mean "if you knit it on the other side, purl it on this side" which makes ribbing, common in cable patterns. If you can link the pattern or a picture or give us its name we can be a little more specific in helping you.